Preparation of lead sulphide mirrors



April 26, 1960 H. L. HEARD PREPARATION OF LEAD SULPHIDE MIRRORS 7 Filed Nov. 18, 1957 Unite States Patent PREPARATION OF LEAD SULPI-IIDE MIRRORS i 5 Harrison L. Heard, London, Untario, Canada, assignor to Canadian Pittsburgh Industries Limited, Toronto, 7 Ontario, Canada Application November 18, 1957, Serial No. 697,085

Claims priority, application Canada July 18, 1957 12 Claims. (Cl. 117-35) The invention relates to a process for depositing a highly reflective film of lead sulphide on a surface. More particularly it relates to the process of depositing such 2,934,454 Patented Apr. 26, 1960 C ce ' achieved inthe process of forming a lead sulphide coating on a surface to be coated in theimprovement which comprises using thioacetamide as the source of sulphide ions. Thus, the preferred process of this invention involves spraying an aqueous solution of suitable lead ions ontoj-the surface to be coated, while at the same time spraying an aqueous solution containing thioacetamide as highly reflective film on a glass surface, whereby to produce what is known as a lead sulphide mirror.

Lead sulphide mirrors are most often used 1n the manufacture of rearview mirrors to be used in automobiles, because such mirrors have a lower deflectivity than conventional silver-coated mirrors. However, lead sulphide'mirrors may be used in the same manner as the conventional silver mirrors, i.e. for use in reflection and decoration. In addition, a lead sulphide film may be applied as a decorative coating to irregularly shaped objects having plastic, enamelled, lacquered, or metallic surfaces.

,In the past, the coating of lead sulphide has been applied as a film to the surface to be coated by contact of such surface with a solution containing lead ions and thiourea under conditions which brought about the. release of the sulphide ions. of placing the surface of the material to be coated in the coating solution, and applying heat to the coating solution whereby the sulphide ions were released from the thiourea. The sulphide ions and lead ions interacted and the film of lead sulphide was deposited on the surface. 7

Such method involves certain intrinsic disadvantages. In the first place, it is extremely time consuming, since the rate of deposition of lead sulphide is directly proportional to the rate of release of sulphide ions from thiourea. The sulphide ions may be released from the thiourea more quickly by using higher temperatures, but such treatment is disadvantageous since it is uneconomical, and furthermore, it is difficult to maintain absolutely even temperatures over the entire surface of the material to be coated with the result that a uniform coating of the lead sulphide on the surface is difiicult to obtain.

A further diificulty attendant with the lack of uniformity of the coating was the formation of pin holes on the surface thereof. In addition, it leads to discoloration of the coating, and to the formation of haphazard interference phenomena.

It is therefore an object of the present invention to disclose a process for preparation of lead sulphide coating in which the coating proceeds at a practical rate even at ordinary room temperatures.

.It is -a further object of this invention to disclose a. process for forming lead sulphide coatings wherein the ingredients are sprayed onto the surface to be coated, whereby to result in a continuous process for forming such lead sulphide coatings.

It is a still further object of this invention to disclose a process for depositing a uniform coating, of controlled thickness, of lead sulphide on a surface.

It is another object of this invention to produce a lead sulphide mirror in which the amount of undesirable pin' holes have been reduced to a or completely eliminated. I v t The process, then, consisted the source of sulphide ions whereby to cause the lead ions and the sulphide ions to interact at the surface to be coated and thereby forming a lead sulphide mirror film in situ.

The present invention is based upon the use of thioacetamide as the source of sulphide ions and in its broadest sense, the invention consists of the use of thioacetamide in the process of depositing lead sulphide mirror films.

Nevertheless, it is generally desirable to use a rate controlling additive in conjunction with the thioacetamide. Althoughit is not desired to limit the present invention to any particular theory, it is believed that the reason for the need for such rate controlling additive is to exercise, a fine control upon the rate of deposition of the lead sulphide. In most applications, then, it will be desirable to have such control on the rate of'deposition. If such control is not desired, it is possible to carry out the process of the present invention using thioacetamide as the source of sulphide ions without the inclusion of any rate controlling additive. The amount of'th'ioacetamide used is not critical, but in general the 35 solution which is the source of sulphide ions will contain an amount of from about 0.5 gram to about 5.0 grams per liter of such solution. Preferably such solution will contain 2.0 grams per liter thereof.

The solvent most generally used is water, since it is most economical.

The ratecontrolling additive is generally a water solu ble, organic compound. Various organic compounds suitable for such purpose include amides, such as urea, thiourea, acetamide and the like, and sugars such as sucrose and the like. In general, the solution which is the source of sulphide ions may contain such additives in an amount of from about 0.001 gram to about 30.0 grams per liter of such solution, a convenient amount being about 5 grams per liter thereof.

The source of lead ions for the solution may be any water-water lead salt. Examples of such water-soluble salts include lead acetate, lead chlorate, lead perchlorate and lead nitrate. The amount of such salts which may be used in the solution is not critical, but in general the solution which is the source of lead ions will contain from a minimum of about 1.0 gramto a maximum of about 30.0 grams per liter of such solution. Preferably, it is desirable to have about 15.0 grams of such lead salts present per liter of such solution.

It appears as if the mechanism of the reaction for the formation of the lead sulphide coating is through the presence of the water-soluble plumbite ion in solution although it is notdesired to limit the present process to any such theory. The plumbite ion is notbelieved to exist in acid solutions, but it is believed that such ion may conveniently be formed by the addition of a strong is necessary in order to enable the lead sulphide to be lead sulphide 11.

-were prepared in the following manner. I taining the lead ions was prepared by dissolying 1 6,5

metal hydroxide, within the above-noted range is added until no insoluble lead hydroxide remains in the solution.-

Normally a pH of not less than about 13 is required.

Although it is not desired to limit the present invention to its use in conjunction with any pre-treatment of the surface which is to be coated with lead sulphide, ithas' been found that more satisfactory results accrue if the.

surface is first subjected to a pro-treatment. Such pretreatment would normally consist of sensitization of the surface with an aqueous solution of stanous chloride. This preliminary sensitization may advantageously be followed by further treatment of an aqueous solution of silver nitrate.

The drawing represents diagrammatically the preferred process of forming a lead sulphide mirror according to,

the. present invention. It is seen from the figure that a sheet of glass is being coated with a film; of lead sulphide It A spray gun or similar type of apparatus having a nozzle 12 is connected to a source (not shown of the particular sulphide ions of the present invention, so

thatIan atomized spray of sulphide ions. 14 is caused to emerge therefrom. Similarly, a spray gun or similar type of apparatus having a nozzle 13, and connected to a source (not shown) of lead ions is arranged so that theatomized spray of lead ions 15 is caused to intersect and commingle with the atomized spray of sulphide ions 14. The rate of spraying of sulphide ions 14 is substantially By reflected light the mirror had a dark, but highly reflective appearance of polished galena. It also showed no interference colours due to varying rates of deposition. By transmitted light, the lead sulphide film was brown and devoid of objectionable pinholes.

Examples II'V 7 Four further tests were carried out, following the general procedure described above the Example I, and in which solutions 1 and 2 were prepared in the same manner as outlined for Example I.

For Example H, solution 1 consisted of 1.6 gm. lead acetate and 46 gm. sodium hydroxide in 1000 cc. of water. Solution 2 consisted of 1 gm. thioacetamide as the source of sulphide ions, in 1000 cc. of water. Thus solution 2 contains no rate-controlling additive.

For Example II, solution I consisted of 1.6 gm. lead acetate and 46 gm. sodium hydroxide in 1,000 cc. of

the same as the rate of spraying of the lead ions l5. I This intermingling permits the interaction of the lead ions and sulphide ions resulting in the deposition of the film; of The spray guns or similar type of apparatus to which the nozzles 12 and '13 are attached are interconnected by a conventional means (not shown) which permit such apparatus to-be moved as a unit in the direction shown by the arrow. In this way, a film of lead sulphide is deposited on the entire surface of the glass 1 0 ,v

and thereby forming a lead sulphide mirror.

The following examples are given to illustrate this. im

vention.

Example I i A pair of solutions containing lead ions and the particular sulphide ions of the present invention respectively Solution 1 congrams of'lead acetate in a minimum amount of water, dissolving 46 grams of sodium hydroxide in a minimum amount of water, mixing the two solutions, and diluting the composite solution to 1 liter with water. Solution 2 containing the sulphide ions was prepared by dissolving 5. grams of thiourea in a minimumamol nt of water, adding 0.5 gram of thioacetamide thereto, and diluting the mixture to 1 liter with water.

The process of coating was generally similar to that described in the figure. The apparatus used was an external mix spray gun having two nozzles; solution 1 emerged from one nozzle and solution 2.cmerged from the other nozzle. The gun was so arranged that the distance from the nozzle to the glass surfacegwas not more than six inches. The two solutions wereatomiz ed by air pressure, and were caused to intermix about '1 inch from the nozzle. The intermixed lead and sulphide ions then come into contact with the surface to be coated and form a thin film of lead sulphide thereon.

The lead sulphide mirror prepared by the above procmay be characterized by the following description.

water. Solutidn 2, consisted, of 2 gm. thioacetamide, as thesource of sulphide ions, and 15 gm. urea, as the ratecontrollingadditive, in 1000 cc. of water.

For ErarnplelV, solution 1 consisted of 1.6 gm. lead acetate and 46 gm. sodium hydroxide in 1000 cc. of water. Solution}. consisted of 2 gm. thioacetamide, as thesourceof sulphide ions, and 0001-0005 gm. sucrose, astherate-controlling additive, in 1000 cc. of, water.

For Example V, solution 1 consisted of 1.6 gm. lead acetate and 46, gm. sodium hydroxide in 1000- cc. of water. Solution 2 consisted of 2 gm. thioacetamide, as the source of sulphide ions,and l5'gm. acetamide, as the raterGOntl'olling additive, in 1000 cc. of water.

' It was found that the lead sulphide mirrors resulting from. EXflmples II V, i.e. produced by the process of the present. invention, had "appearances similar to the lead lack ofv interference colours by reflected light.

What I claim is: u r 1, Inthe proccssfor producing a film of lead sulphide on. a surface to bei'coated by contacting said surface with,

a source of lead ions, and a source of sulphide ions, the improvement which comprises contacting the surface ofasheet of glass with an aqueous solution of lead ions, and with an aqueous solution of thioacetamide as the sourceof, sulphide ions, whereby to form a lead sulphide m rror.

2. In the process for producing a film of lead sulphide one surface to be coated by contacting said surface with a source of lead ions and a source of sulphide ions, the improvement which comprises spraying an aqueous solution of lead i ons generally in the direction of a glass sheet,

spraying an aqueoussolution of thioacetamide as thev improvement which comprises contacting the surface of a sheet of glass with an aqueous solution of lead acetate, and;v with an aqueous: solution of thioacetamide as the source of sulphide ions, whereby to form a lead sulphide mirror,

4." A process of preparing a lead sulphide mirror, which comprises contacting the surface of a sheet of glass with an aqueous solution containing lead acetate and sodium-hydroxide, and with an aqueous solution of thioacetamide asthe sourceof sulphide ions.

5;. A process of preparing a lead sulphide mirror which comprises spraying an aqueous solution containing lead acetate and sodiumv hydroxide generally in the direction of a glass sheet, spraying an aqueous solution of thioacetamide as the source of sulphide ions also generally in the direction of a glass sheet to cause the two solutions to intermingle, and causing the intermingled solutions 7 to contact said glass sheet.

from about 15.0 grams per liter to about 90.0 grams I per liter of sodium hydroxide, and with an aqueous solution of thioacetamide as the source of sulphide ions.

7. A process of preparing a lead sulphide mirror which comprises spraying an aqueous solution containing from about 1.0 gram per liter to about 30.0 grams per liter of lead acetate and from about 15.0 grams per liter to about 90.0 grams per liter of sodium hydroxide generally in the direction of a glass sheet, spraying an aqueous solution of thioacetamide as the source of sulphide ions also generally in the direction of a glass sheet to cause the two solutions to intermingle, and causing the intermingled solutions to contact said glass sheet.

8. A process of preparing a lead sulphide mirror, which comprises contacting the surface of a sheet of glass with an aqueous solution containing lead ions and sodium hydroxide, and with an aqueous solution containing from about 0.5 gram per liter to about .0 grams per liter of thioacetamide as the source of sulphide ions.

9. A process of preparing a lead sulphide mirror which comprises spraying an aqueous solution containing lead ions and sodium hydroxide generally in the direction of a glass sheet, spraying an aqueous solution containing from about 0.5 gram per liter to about 5.0 grams per liter of thioacetamide as the source of sulphide ions also generally in the direction of a glass sheet to cause the two solutions to intermingle, and causing the intermingled solutions to contact said glass sheet.

10. A process, of preparing a lead sulphide mirror, which comprises contacting the surface of a sheet of glass with an aqueous solution containing from about 1.0 gram per liter to about 30.0 grams per liter of lead acetate and from about 15.0 grams per liter to about 90.0 grams per liter of sodium hydroxide, and with an aqueous solution containing from about 0.5 gram per liter to about 5.0 grams per liter of thioacetamide as the source of sul phide ions. 1

11. A process of preparing a lead sulphide mirror which comprises spraying an aqueous solution containing from about 1.0 gram per liter to about 30.0 grams per liter of lead acetate and from about 15.0 grams per liter to about 90.0 grams per liter of sodium hydroxide generally in the direction of a glass sheet, spraying an aqueous solution containing from about 0.5 gram per liter to about 5.0 grams per liter of thioacetamide as the source of sulphide ions also generally in the direction of a glass sheet to cause the two solutions to intermingle, and causing the intermingled solutions to contact said glass sheet.

12. The process for producing a coating of a mirrorlike film of lead sulphide on a surface, which process comprises contacting said surface with a source of lead ions in solution having a pH of about at least 13 while substantially simultaneously contacting said surface with a solution of thioacetamide as a source of sulphide ions.

References Cited in the file of this patent UNITED STATES PATENTS Colbert et a1. Dec. 3, 1946 

1. IN THE PROCESS FOR PRODUCING A FILM OF LEAD SULPHIDE ON A SURFACE TO BE COATED BY CONTACTING SAID SURFACE WITH A SOURCE OF LEAD IONS, AND A SOURCE OF SULPHIDE IONS, THE IMPROVEMENT WHICH COMPRISES CONTACTING THE SURFACE OF A SHEET OF GLASS WITH AN AQUEOUS SOLUTION OF LEAD IONS, AND WITH AN AQUEOUS SOLUTION OF THIOACETAMIDE AS THE SOURCE OF SULPHIDE IONS, WHEREBY TO FORM A LEAD SULPHIDE MIRROR. 